Materials that can both conduct and

insulate against the flow of electricity are

of great interest for use as components in

advanced electronic devices as well as for

fundamental condensed-matter studies.

Nearly 50 years ago, the transition-metal

oxide V2O3 was found to qualify as such a

material, being a conductor (i.e., metallic)

above 160 K and an insulator below.

Despite progress in methods to model

these observations theoretically, the lack

of momentum-resolved photoemission

data to verify the theories meant that the

origins of this phenomenon ultimately

remained a mystery. Now, researchers

report on angle-resolved photoemission

spectroscopy (ARPES) measurements

performed on samples of V2O3. The results

overturn a decade-old theory about

metal–insulator transitions (MITs) in this

material and provide a spectroscopic

benchmark test for future models.

Initially, V2O3 was thought to be a real-

world example of an ideal Mott MIT, in

which the insulator state arises as a result

of Coulomb repulsion between electrons.

However, this was quickly challenged on

the grounds that it didn’t take into account

the material’s multi-orbital electronic

structure: each trivalent vanadium ion

has two 3d electrons distributed between

two overlapping d-orbitals (a1g and eπg).

In the late 1990s, the integration of

density functional theory (DFT) with

dynamical mean-field theory (DMFT)

gave real hope of solving the mystery

within a realistic multiorbital calculation.

Bulk-sensitive angle-integrated photo-

emission provided a basic confirmation

of the DMFT theory, but was not able

to discriminate between the details of

different DFT+DMFT theories.

A series of DFT+DMFT studies in the

early 2000s coalesced in 2007 around a

particular model in which Coulomb

Science In Progress

The metal-to-insulator transition in V2O3 (vanadium sesquioxide) has a long history of explanatory models being proposed, generally accepted, and overturned by new experiments. Ever since reports revealing the full doping-pressure-temperature phase diagram for V2O3 were published in 1969–1973, successive generations of condensed-matter physicists have been determined to solve the mystery of why V2O3 behaves the way it does. One reason for this was that it seemed to be the perfect examplar of a type of insulator proposed by Nevill Mott in 1949. A “Mott insulator” is a material that’s expected to be a conductor, but isn’t, despite having a half-filled outer electron orbital (a characteristic of metals). The repulsion between the electrons in this orbital causes them to localize, creating an energy gap between filled localized states (where the electrons are immobile) and a conduction band (where they can move around). Subsequent models incorporated more complexity, but remained unconfirmed by empirical data. In this work, Lo Vecchio et al. provide such data—an important check on a decade-old picture, suggesting that there is still more work to do.

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360 eV 320 eV 280 eV

(a) V2O3 crystal structure showing the hexagonal unit cell (black lines) and the primitive rhombohe-

dral cell (blue lines), with four vanadium atoms (blue circles) in the center. (b) Brillouin zone for the

metallic-phase rhombohedral unit cell. (c) Wide angle-dependent slices of the Fermi surface of V2O3

(as indicated by dashed red lines in b) at T = 200 K (metallic phase). A triangular-shaped feature

reverses direction above and below the Brillouin zone center.

ARPES Overturns V2O3 Metal-to-Insulator Theory

MATERIALS SCIENCES

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repulsion forces drive the splitting of the

a1g and eπg states in the metallic phase to the

brink of an insulating gap near the MIT.

Small structural perturbations from

temperature changes and doping would

then be needed to push the system over

the transition. The model’s predictions

included a Fermi surface featuring a single

a1g electron pocket, as well as completely

filled eπg bands. Over time, however, technical

advances in the DFT+DMFT calculations

began to show discrepancies with the 2007

model, setting the stage for the next logical

development: measurement of the momen-

tum-resolved band structure of V2O3.

Scientists at the ALS performed

ARPES experiments on single-crystal

V2O3 samples using the 2009 configura-

tion of Beamline 7.0.1 (the ARPES program

has since moved to Beamline 4.0.3). The

wide-angle detection capability of the

beamline’s electron spectrometers

was important for characterizing the

weak-contrast, angle-dependent,

band-structure changes in the rather

broad features inherent in the V2O3

spectra. The measurements were taken

at a temperature of 200 K, fully in the

metallic phase.

The measurement of a triangular

Fermi surface with a hole-like band

dispersion—as opposed to an electron

pocket—was the crucial observation

directly contradicting the 2007 theory.

While the key Fermi surface and band

dispersion data were acquired at a photon

energy of 180 eV, the wide photon-energy

range of Beamline 7.0.1 (80–900 eV) also

allowed the verification of the triangular

Fermi surface feature at more bulk-sensi-

tive ranges, thus helping to prove the bulk

origin of features that were measured at

lower, more surface-sensitive, energies.

To support their findings and gain

greater insight, the researchers performed

band calculations using DFT+U methods

(U is a parameter accounting for the

Coulomb repulsion between electrons).

A match was found for the specific half-

filling of the eπg band observed in the data.

The calculations also gave rise to a Fermi-

surface contour with an open-tipped

triangular feature very similar in size and

shape to that obtained via the ARPES

experiments. In the future, analysis using

the more-sophisticated DMFT will be

needed to fully explain the details of the

experimental data and to provide a new

explanatory mechanism for the metal–

insulator transition in V2O3.

348 • 03/17

Publication about this research: I. Lo Vecchio, J.D. Denlinger, O. Krupin, B.J. Kim, P.A. Metcalf, S. Lupi, J.W. Allen, and A. Lanzara, “Fermi Surface of Metallic V2O3 from Angle-Resolved Photoemission: Mid-level Filling of eπ

g Bands,” Phys. Rev. Lett. 117, 166401 (2016). doi: 10.1103/PhysRevLett.117.166401

Research conducted by: I. Lo Vecchio (Berkeley Lab), J.D. Denlinger and O. Krupin (ALS), B.J. Kim (Max Planck Institute for Solid State Research, Germany), P.A. Metcalf (Purdue University), S. Lupi (Sapienza University of Rome), J.W. Allen (University of Michigan), and A. Lanzara (Berkeley Lab and UC Berkeley).

Research funding: National Science Foundation. Operation of the ALS is suppoerted by the U.S. Department of Energy, Office of Basic Energy Sciences.

Published by the ADVANCED LIGHT SOURCE COMMUNICATIONS GROUP

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The a1g (left) and eπg (center) molecular orbitals of V2O3, represented graphically in 3D and in

band-structure (“spaghetti”) plots (right).

Comparison of the triangular Fermi-surface feature obtained from ARPES data (left) and from DFT+U

calculations (right).

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